Do Sharks Feel Pain?

Of all the stirring tales of maritime lore, few are as spectacular and
chilling as the shark feeding frenzy. During a full-blown frenzy, sharks are
reputedly driven 'mad' by an insatiable blood lust. According to whalers'
wide-eyed but seemingly earnest accounts, in the throes of violent gluttony,
sharks feeding at a whale carcass are undeterred from their bloody feast by
even the most serious of injuries - such as being savagely stabbed and
sliced by flensing knives (used by whalers to strip blubber from whale
carcasses) or bitten repeatedly by other sharks. There are even reports of
sharks that have been split in two by whalemen or disemboweled by other
sharks continuing to feed, apparently ignoring their mortal wounds as though
insensate to them. From vivid stories such as these, it came to be believed
that sharks do not feel pain.

A precise neurological understanding of pain remains elusive. We do know
that, in vertebrates, sensory neurons (nerve cells) penetrate most tissues
and are responsible for relaying information to the central nervous system
from the rest of the body. In mammals, two morphologically distinct
populations of sensory nerve cells have been identified: relatively thick
neurons having axons sheathed with myelin (a fatty, electrically insulating
layer) and thinner neurons without. These thinner, unmyelinated neurons
respond only to stimuli sufficiently intense to cause tissue damage and are
termed nociceptors. In humans, tissue damage activates nociceptors, evoking
a sensation of pain. Conversely, people who are born insensitive to pain
virtually lack such nociceptors. Therefore, nociceptors are essential to the
ability to perceive pain.

A fascinating 1993 paper by Australian neurophysiologists Peter Snow,
Mark Plenderleith, and Layne Wright examined the sensory neurons of three
species of elasmobranchs. Electron micrographs (images from an electron
microscope) were used to count the number of myelinated and unmyelinated
neurons in a key portion (the dorsal root ganglia) of the nervous systems of
the Giant Shovelnose Ray (Rhinobatos typus), the Honeycomb Whip-tail
Ray (Himantura uarnak), and the Blackfin Reef Shark (Carcharhinus
melanopterus). Snow and his co-workers found that less than 1 percent of
the neurons of both ray species were unmyelinated, while from 14 to 38
percent of the shark neurons were unmyelinated. In humans, by comparison,
about 50 percent of neurons are unmyelinated.

Snow and his co-workers concluded that elasmobranchs lack the neuronal
machinery absolutely essential for the perception of pain. The researchers
went on to speculate that, for sharks and rays, the ability to perceive pain
might have little relevance to survival. It is difficult to imagine how this
could be. We humans rely on the sensation of pain to indicate that our
bodies are not functioning properly, so that we might take action to limit
the damage caused by injuries. It is remarkable that sharks, which have
proven to be extremely sensitive to many other stimuli, would be insensate
to sensory cues that could warn of injuries that could prove
life-threatening in a dangerous world.

Yet, according to the available neurophysiological and behavioral
evidence, sharks do not seem to feel pain. If sharks do feel pain, then
their mechanism and experience of this noxious sensation must be very
different from our own.